The field of the invention is the manufacture of expandable styrene polymers by suspension polymerization and the present invention is particularly concerned with the control of the size and shape of the expandable styrene beads produced.
The state of the art of expandable polystyrene may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966), under the section entitled "Foamed Plastics", pages 847-884, particularly pages 852, 853 and 855, where polystyrene is disclosed and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 120 where prior art self-extinguishing polystyrene foams are disclosed, as well as reference to U.S. Pat. Nos. 4,228,244 and 4,337,319, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,228,244 is incorporated by reference to show the process steps necessary to manufacture molded foam bodies. According to this process, the fine particulate styrene polymers are first heated by means of steam or hot gases to temperatures above their softening points, whereby foaming takes place into discrete particles. This procedure is denoted as pre-foaming. The pre-foamed polystyrenes are then temporarily stored and later further expanded by additional steam heating in a pressure-resistant mold whereby the particles weld into one another to a molded body corresponding to the inside cavity of the mold. This second procedure is denoted as final foaming. The molded object, after final foaming, is cooled inside the mold until the inside temperature drops below the softening point. When the molded object is prematurely removed from the mold, the object deforms. As foam plastics are good insulators, relatively long cooling times are required to cool the mold. The time interval allowing the earliest removal of the molded object without deformation is ordinarily called the "minimum mold dwell time."
U.S. Pat. No. 4,337,319 is incorporated by reference to show the preparation of self-extinguishing, fine particulate, expandable styrene polymers for the manufacture of molded articles.
The state of the art of controlling the size and shape of expandable styrene beads during bead polymerization or suspension polymerization may be ascertained by reference to U.S. Pat. Nos. 3,222,343 and 4,036,794; British Pat. No. 1,226,959; French Pat. No. 2,079,991; West German Published Application No. 2,510,937; the Trommsdorf and Meunster article in Schildknecht: Polymer Processes, Vol. 29, pp. 119-120; Houben-Weyl, Methoden der Organischen Chemie, 4th Edition, Vol. XIV, part 1, pp. 422 and 425; the Winslow and Matrayek article in Industrial and Engineering Chemistry, Vol. 43 (1951), page 1108, and the article by H. Wenning entitled "On the Colloidal Chemistry of Bead Polymerization" as published in Kunststoffe-Plastics, Vol. 5, (1958), pp. 328-340, the disclosures of which are incorporated herein by reference.
Essentially expandable or foamable styrene polymers are produced by the process of bead polymerization or suspension polymerization in the aqueous phase. Conventional present day suspension stabilizers are water-soluble organic polymers, which are designated as protective colloids. Again, fine particulate powders, for instance calcium or barium sulfate or calcium phosphate, are useful to stabilize the suspension. Such stabilizer systems are known as Pickering stabilizers. A list of commercially employed protective colloids can be found for instance in the Trommsdorf & Muenster article in Schildknecht: Polymer Processes, Vol. 29, pp. 119-120.
The following are reasons why the selection of suitable protective colloids assumes importance:
(1) Controlling narrow grain distributions of defined sizes
Depending on bead size, foamable styrene polymers are used for different applications: coarse beads (2.5 to 0.8 mm) are used in the manufacture of insulating panels and finer fractions (0.8 to 0.4 mm in diameter) are used for making packing materials. It is, accordingly, necessary that the beads always be produced within the desired range of grain size in adequate amounts, that is, in high yields. The production of excessively large or excessively small grains is maintained as low as possible.
(2) Low internal water content of beads
It is known that a certain amount of water is always included in the beads in the conventional suspension polymerization. Polymers with a low content of included water evince a uniform foam structure when foamed, whereby the thermal insulation of the foam panels is positively affected. Therefore the lowest possible content in included water (inner water) is sought.
(3) Spherical shape of beads
Due to improved processing, deformed beads are sought when suspension polymerizing of styrene is carried out free of expanding agents. However, the beads should be as spherical as possible when expandable styrene polymers are being produced.
(4) Sufficient suspension stability over the entire polymerization cycle
The suspension used in the production of expandable styrene polymers is even more unstable than that of styrene polymers free of expanding agents. Considering the present day conventional sizes of reactors, up to 100 m.sup.3, the loss of one batch represents substantial economic loss. Therefore, the phase separation should proceed so slowly, in the event of malfunction, that enough time remains to add a polymerization inhibitor.
None of the suspension systems known to the prior art meet all of the above requirements simultaneously. There have been many attempts to find a suitable way to meet all four requirements at the same time. As shown by the state of the art, these endeavors, however, have remained unsuccessful.
U.S. Pat. No. 4,036,794discloses a method wherein suspension stabilizers are used which were prepared by radical polymerization from styrene in the presence of polyvinyl pyrrolidone.
West German Published Application No. 2,510,937 discloses a method wherein the initially low viscosity system is slightly stabilized with tricalcium phosphate and the post stabilization takes place with an aqueous solution of polyvinyl pyrrolidone several hours later.
Both methods strive to produce styrene polymers with low contents of inner water. However, these methods incur the drawback that the polymer grain size is determined by the time of addition of the organic protective colloid.
It is difficult to precisely determine the polymerization degree with heterogeneous mixtures as are present in suspension polymerization. However, precise knowledge of the degree is required for reproducibly adjusting the grain spectra, because the bead size depends on the particular viscosity of the polymerizing phase at which the protective colloid is being added. Furthermore the polymerizing system stays in an unsafe operational condition for about 2 hours, which represents a special drawback when using large scale reactors. Any malfunction, for instance agitator failure, especially failure at the onset of polymerization, can result in reactor destruction.
British Pat. No. 1,226,959 discloses using two protective colloids, namely polyvinyl alcohol with a different degree of hydroxylation in order to obtain uniformly large, round beads. As shown by the examples of British Pat. No. 1,226,959, this requires selecting the ratio of styrene to water in such an unfavorable manner that the method becomes uneconomical. This method makes no contribution to properly controlling the grain size of beads.
As mentioned initially, inorganic, water-insoluble powders are also used as suspension stabilizers. The use of calcium phosphates is most common. As a rule these inorganic compounds are used in combination with small amounts of emulsifiers or surfactants as disclosed in Houben-Weyl, Methoden der organischen Chemie, 4th Edition, Vol. XIV, part 1, Macromolecular Substances, p 425. However, these systems are used in a limited way as compared to the organic protective colloids because reproducible handling and problem-free suspension polymerization are possible only within a restricted range. The Houben-Weyl reference states in this respect, on page 422, last paragraph, lines 6 through 8, as follows: "It is next to impossible to state conditions under which a pulverulent dispersant would be suitable for a wider application". Precise dosage must be observed when inorganic compounds are combined with surfactants because coagulation results from both excessive and insufficient doses of the surfactant.
French Pat. No. 2,079,991 further discloses how to change the bead shape both by the amount of the suspension medium (protective colloid), or by varying the phase ratio of the aqueous to the organic phases and by using a mixture of an organic protective colloid and an inorganic suspension stabilizer. Spherical beads are not necessarily obtained from this procedure, nor beads with a low inner water content, because the dispersing agent is not added to the aqueous phase before polymerization. When the dispersing agent is added at the beginning of the polymerization, the grain size cannot be reproducibly set.
Again, the similar method of U.S. Pat. No. 3,222,343 fails to meet the four conditions listed above.